I. Field of the Invention
This invention relates to measuring the power distribution of a nuclear reactor fuel element. In particular, the invention relates to a method and passive apparatus for analysis of power distribution history through the measurement of residual radiation along a fuel element.
In order to verify the accuracy of nuclear calculations and to determine if anomalies occurred in the power production of the nuclear fuel, it is desirable to map the recent power distribution of a reactor and of the individual nuclear fuel elements. It has been found that the deposition of certain fission products in a fuel element or rod, specifically radioactive lanthanum-140 which is a fission product of barium, does not migrate within the fuel element and thus is representative of the recent reaction history of the nuclear fuel. The reaction history is directly correlated to the most recent power production, a consequence of the relatively short half-life of the parent reactant, barium-140. Thus, measurement of the radiation emission of lanthanum-140--which must be discriminated from other residual radiation--can be used to construct an accurate map of reactor power distribution.
II. Description of the Prior Art
In the past, gamma radiation from fuel elements has been measured by scanning each fuel element with collimated radiation detector capable of sensing radiation and of discriminating levels of energy. Typically, a sodium iodide scintillation type sensor or a germanium-lithium solid state sensor is used to detect gamma radiation, together with appropriate electronic and signal processing apparatus. There is, however, a substantial danger of exposure of the detector operator to dangerous levels of gamma radiation during measurement of the residual radiation of spent, yet radiating, fuel elements, since the detectors have in the past been manipulated by the operator in close proximity to the fuel elements. Therefore, a shielded detector system has been a requirement of prior art systems. Water has been the usual means of shielding the fuel element to be measured, so the prior art sensors were generally adapted to operate underwater. In summary, according to prior art methods, to determine a distribution history of a fuel element, each element was removed from the reactor, immersed in a shielding medium and slowly scanned by a sensor which generated exposure data one point at a time along one fuel element at a time. This technique has required extensive correction for radiation decay to account for scan time differences at different spatial locations along the fuel element.